1. Field of the Invention
The present invention relates to an optical communication network, and more particularly to a passive optical network (PON).
2. Description of the Related Art
Wavelength division multiplexing passive optical networks (hereinafter, referred to as WDM-PON) provide ultra high-speed broadband communication service using specific wavelengths assigned to each subscriber unit. WDM-PONs can ensure the secrecy of communication, can easily accommodate special communication services required from each subscriber unit or enlargement of channel capacity, and can easily increase the number of subscriber units by adding specific wavelengths to be assigned to new subscribers.
However, in spite of the advantages described above, the WDM-PON has not yet been put to practical use. This is because a central office (CO) and a plurality of optical network units (ONUs) in the WDM-PON require both light sources having specific oscillation wavelengths and additional wavelength stabilization circuits for stabilizing the wavelengths of the light sources. This puts a heavy economic burden on the subscribers. In order to construct an economic WDM-PON, some conventional WDM-PON have tried using a fabry-perot laser wavelength-locked with inherent light or a reflective semiconductor optical amplifier as a WDM light source, which allow a spectrum sliced broadband light source to facilitate wavelength management.
Generally, the conventional WDM-PON uses a double star structure in order to minimize the length of optical line. A central office and a remote node (RN) installed at an area adjacent to optical network units are connected to each other through one main optical fiber (MOF). The remote node and each optical network unit are connected to each other through a separate distribution optical fiber (DOF). Multiplexed downstream optical signals are transmitted to the remote node through the main optical fiber. The multiplexed downstream optical signals are demultiplexed by a wavelength division multiplexer installed in the remote node and the demultiplexed signals are transmitted to the optical network units through the distribution optical fibers. The upstream optical signals output from the optical network units are transmitted to the remote node and multiplexed by the wavelength division multiplexer. The multiplexed signal is transmitted to the central office.
In such WDM-PON, large amounts of data are transmitted at high speed through wavelengths assigned to each optical network unit. Accordingly, when an abnormality (such as malfunction or deterioration) of an upstream light source or a downstream light source or an abnormality (such as cut or deterioration) of a main optical fiber or distribution optical fiber occurs, the transmitted data may be lost even if the abnormality only occurs for a short time. Accordingly, such an abnormality must be quickly detected and be corrected.
However, when the direct optical line between the central office and the optical network units is cut, the central office and the optical network units cannot report the existence or absence of abnormality to each other. For this situation, a separate low speed communication line may be provided. However, in order to install the separate low speed communication line the central office and each optical network unit, additional cost is required and investment is required for continuously managing and supervising the separate low speed communication line. In addition, in order for the central office and each optical network unit to communicate with each other and check the existence or absence of abnormality through the separate low speed communication line, and to report a manager of the abnormality occurrence, a separated time period is required. As a result, a communication interruption state between the central office and each optical network unit is extended by the time period.
It is also necessary to develop a monitoring method, which can quickly detect an abnormality of an upstream light source or a downstream light source, or an abnormality of a main optical fiber or a distribution optical fiber, and directly report the manager of the existence or absence of abnormality, and a correction method.
The abnormality of the downstream light source or the abnormality of the main optical fiber connecting the central office to the remote node can be monitored by the central office which manages the operation state of the downstream light sources and the received state of all upstream optical signals. For example, when it is assumed that an abnormality does not occur at each distribution optical fiber connecting the remote node to each optical network unit, the state of the upstream light source installed at each optical network unit may be monitored from an upstream optical signal received in an upstream optical receiver installed at the central office. However, when an abnormality occurs at one distribution optical fiber, since the central office cannot receive an upstream optical signal progressing to the distribution optical fiber, the state of the upstream light source cannot be monitored.
Accordingly, in the WDM-PON, a method is required, which can monitor an abnormality of the distribution optical fiber. Further, a monitoring method is required, which can distinguish and recognize an abnormality of the upstream light source and an abnormality of the distribution optical fiber. Furthermore, when an abnormality has occurred at the upstream light source or the distribution optical fiber, a method capable of healing the abnormality is required.